Betulin is a natural pentacyclic triterpene, which is inter alia contained in varying amounts in the outer layers of the bark of birch trees (Betula sp.). Betulin is also an important commodity, particularly in cosmetic and pharmaceutical industry, for its various biological effects (Dzubak, P.; Hajduch, M.; Vydra, D.; Hustova, A.; Kvasnica, M.; Biedermann, D.; Markova, L.; Urban, M.; Sarek, J. Nat. Prod. Rep. 2006, 23, 394-411). Betulin is used either as such or in the form of its derivatives and compounds. Birch bark is easily available in huge amounts, because it is a waste product in paper and cellulose production in northern countries, where almost exclusively birches are processed (Jääskeläinen P.: Paperi-ja Puutavaralehti 1981, 10, 599-603). Daily, 40 tons of crude birch bark are produced worldwide, this bark is at present used only as a cheap fuel 5-7 USD/ton, 7-11 MJ/kg (Krasutsky P.: Nat. Prod. Rep. 2006, 23, 919-942).
The rigid skeleton of betulin is composed of 30 carbon atoms and is substituted only with two hydroxy groups, thus, as a lipophilic organic compound, it is almost insoluble in water and for the extraction thereof from a natural material, it is necessary to use other solvents. Most often, various organic solvents are used (e.g. butan-1-ol in RU2234936, toluene in RU2192879, petrolether in RU2194120, or lower alcohols—methanol, ethanol, isopropyl alcohol in RU2674867), in which betulin is more soluble at a higher temperature and after concentrating the extract, it can crystallize from the extract. In these solvent extractions, usually a ground birch bark is extracted with a hot solvent, the extract is then concentrated and after cooling down, crude betulin crystallizes from the extract. The extractions are often combined with washing with lyes in order to remove admixtures of acids (US2003153776, RU2270202, RU2270201). Chemical modification of the birch bark, either by oxidation agents or by acids, is used for the preparation of various betulin derivatives and the resulting derivatives are directly extracted by solvents such as diethyl ether or dichloromethane etc., which are present in the reaction mixture together with the bark (U.S. Pat. No. 6,280,778). This method firstly does not allow to obtain betulin from the birch bark, and then, it is very non-ecological and non-economical, because it uses substantial amounts of toxic organic solvents and produces a waste birch bark contaminated with oxidative and other process waste products. Another modern possibility to isolate betulin from birch bark is its supercritical extraction with carbon dioxide or with an admixture of methanol, ethanol or acetone (CN1634972, US2005158414), or eventually, these extraction techniques are combined with an activation of the bark by steam (RU2074867) or superheated steam (Kuznetsov B. N., Levdanski V. A., Polezhaeva N. I.: Khim. Rastit. Syr'ya, 2004, 2, 21-24), optionally in combination with ultrasound (RU2264411). Apart from the extraction methods of betulin isolation, also a sublimation method is known (Pakdel H., Murwanashyaka J. N., Roy C.: J. Wood CHem. Technol. 2002, 22, 174-155), which is characterized by the use of high vacuum and high temperatures, which provides a low yield of betulin. Only one patent document US2005158414 deals with the concentration of the birch bark component containing mainly the outer layers of the birch stem, which is based on a pneumatic separation by compressed air or on a separation through a mesh.
At present, a substantial ecological pressure is put on technologies in general, and pharmaceutical technologies in particular, not to harm the environment, i.e. not to use toxic substances and not to produce a toxic waste. Of course, industrial technologies must be ecological and economical at the same time. The above-mentioned solvent extraction methods of betulin production from the birch bark do not fulfil particularly the ecological criteria, because they use large volumes of toxic and flammable organic solvents, which cannot be fully regenerated, hence, at once waste solvents are produced and the bark after the extraction is soaked with these solvents. Furthermore, the thus obtained crude betulin has the purity of only 75-85%, contains a large amount of ballast admixtures (triterpenoids, fatty acids, etc.), is coloured and it is necessary to refine it before a further use, which is expensive. Supercritical (SC) methods fulfil high ecological requirements only partly, because in order to achieve sufficient extraction yield, it is necessary to use a mixture of carbon dioxide with organic solvents (e.g. ethanol, methanol; US2005158414) and relatively long extraction times. The betulin isolation from the extract and organic solvent recycling is thereby complicated. The SC extract has the purity of about 80%, contains a substantial fraction of low-molecular substances—fats, waxes, fatty acids, suberinic acids, triterpenoids and also dyestuffs. With regard to high operational costs for the SC extraction and the necessity of refining the SC extract, this method is very costly.
For the above listed reasons, it is apparent that the above-mentioned methods of production of the important triterpene betulin are not optimal, because they do not fulfil high ecological or economical requirements for modern technological processes. The aforementioned disadvantages are solved by the present invention, which connects the supercritical extraction with a chemical modification of the raw material.